Method and system for cleansing water from oil

ABSTRACT

A method and apparatus for extracting oil from water, particularly spillage water from gasoline stations and water that emanates from vehicle-washing plants. The method and apparatus utilize filters to which contaminated water from the vehicle-washing plant and other contaminated water is passed. The water is cleansed in one or more mutually sequential filters arranged in the flow direction of the water. The water is cleansed by first causing the water to flow through a collection filter in which oil and coarse particles are collected in a first cleansing stage. The water from the collection filter is passed through a fine filter adapted to lower the oil content of the water to a level below about 50 to 70 mg/l in a second cleansing stage. The water from the fine filter then flows through a membrane filter that is adapted to filter-off at least 75% of the oil remaining in the water during a third cleansing stage. At least a substantial part of at least the water cleansed in the membrane filter is recycled back to the vehicle-washing plant while any residual part of that water is discharged to a municipal sewage network.

BACKGROUND OF THE INVENTION

The present invention relates to a method and to a system for cleansingwater from oil, and more particularly for cleansing water spillage ingasoline or petrol stations.

The present invention is concerned more particularly with cleansingvehicle wash water with respect to its oil and heavy metal content.

DESCRIPTION OF THE RELATED ART

Many countries stipulate that sewage water from vehicle-washingfacilities shall be essentially free from oil, or at least have a verylow oil content. Under Swedish regulations, the oil content of sewagewater must be below 50 mg/l as from the year 1997.

One known method of extracting oil from oil-contaminated waste water isto feed the water into a flocculating chamber connected to a slimeseparator located at the gasoline station. This method achieves apurification of 75%-85%. However, the method is not sufficientlyeffective to achieve the aforesaid purification level, and the waterleaving the purification plant will typically have an oil concentrationof 150 mg/l or higher.

Filters with which the oil content of oil-contaminated water can begreatly reduced are known to the art. The membrane filter is one suchfilter type. Such filters are expensive, however. Furthermore, membranefilters have a relatively low capacity with regard to the amount of oilthat is extracted, and consequently it would be necessary to connect alarge number of membrane filters in parallel in order to cleansesatisfactorily sewage water that derives from vehicle-washing plants,this water normally having an oil concentration as high as 150 mg/l.

It would appear that although waste water cleansing or purifyingtechniques are known to the art, the techniques are not economicallyviable with regard to vehicle-washing plants, where a typical waterconsumption is about 400 liters with each vehicle wash.

The present invention solves this cleansing problem and provides arelatively inexpensive arrangement which has a very high cleansingefficiency.

SUMMARY OF THE INVENTION

The present invention thus relates to a method of extracting oil fromwater, particularly spillage water from gasoline stations, includingwater that emanates from vehicle-washing plants, having filters to whichcontaminated water from the vehicle-washing plant and possibly othercontaminated water spillage is passed. The water is cleansed in one ormore mutually sequential filters in the flow direction of the water bycausing the water to flow through a collection filter in which oil andcoarse particles are collected in a first cleansing or purifying stage.The water from the collecting filter is caused to pass through a finefilter which is adapted to lower the oil content of the water to a levelbelow approximately 50-75 mg/l in a second cleansing or purifying stage.The water from the fine filter is caused to flow through a membranefilter which is adapted to filter-off at least 75% of the oil remainingin the water in a third cleansing or purifying stage. At least asubstantial part of at least the water cleansed in the membrane filteris recycled back to the vehicle-washing plant while any residual part ofsaid water is discharged to a municipal sewage network.

The present invention also relates to apparatus for carrying out theinventive method.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to twoexemplifying embodiments thereof and also with reference to theaccompanying drawing.

FIGS. 1 and 2 are each flowsheets that show a respective embodiment ofthe present invention in which conventional fluid system symbols havebeen used. Consequently, not all units will be described in thefollowing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a first embodiment of the invention, wherein thenumeral 1 identifies a feed pump, numeral 2 identifies a flocculatingchamber, numeral 3 identifies a flocculating agent metering tank,numeral 4 identifies a collection filter and numeral 5 identifies a finefilter.

The collection filter 4 is of a kind in which large particles, such asclumps of oil from the flocculating chamber, are extracted mechanicallyand remain in the filter. The filter may include metal gratings, grids,or the like. The collection filter is thus emptied at given timeintervals.

The fine filter 5 is of a kind that can be cleansed by delivering cleanor fresh water thereto from the municipal or local water mains. Onesuitable known type of filter in this regard is a sand filter. In thiscase, clean water or fresh water is passed through a conduit means 6 andthrough the filter 5 and from these to a drainage tank 8 via a three-wayvalve 7. This water is circulated back to the flocculating chamber 2through a conduit mean 9 and the feed pump 1.

A second embodiment of the invention shown in FIG. 2 lacks the provisionof a flocculating chamber and drainage tank 8. The system of apparatusaccording to this second embodiment will also preferably lack theprovision of a sludge separating tank 32. In this case, water isconducted directly from the washing plant 37 to the collection filter 4through a conduit 38. The system of apparatus is the same as that shownin FIG. 1 in other respects.

In the case of this second embodiment, the fine filter 5 includes a massbased on sawdust that has been treated to render the mass hydrophobic,whereby the mass is able to absorb petroleum products without taking-upany appreciable amount of water. The contaminated water is caused topass through the mass. Such a mass is retailed by Prolup Absorbent AB,Atvidaberg, Sweden. The mass can be made hydrophobic, by treating thebasic material with Teflon. This provides a product which will rejectwater while effectively soaking up petroleum products.

According to one preferred embodiment, the mass is admixed with at leastone of the following substances: active carbon, aluminum selenite, orsynthetically produced selenite. These substances bind heavy metals,therewith effectively purifying the water of such metals.

The aforesaid mass is replaced at pre-determined intervals and the usedmass is disposed of together with normal garbage that is later burned orcombusted. The mass will have a relatively high calorific value, due toits petroleum product content.

Applicable to both embodiments is that the water cleaned in the finefilter 5 is passed through a conduit means 10 to a buffer tank 11. Thewater is passed from the buffer tank 11 through a conduit means 12 to atleast one membrane filter 13 in which it is finally cleansed with regardto oil. Membrane filters are of a know kind, with which liquid is causedto pass through, e.g., a polyamide material that has small pores. Thepore size is typically between 20-80 nanometers. The cleansed part ofthe water is passed further through a conduit means 14, while theremaining part is passed back to the buffer tank, through a conduitmeans 15.

The membrane filter 13 is cleaned by pumping an appropriate knownwashing liquid therethrough from a tank 17 with the aid of a pump 16,wherein the washing liquid is preferably passed through the membranefilter in a direction opposite to the flow direction prevailing in thewater-cleansing process. After having passed through the membranefilter, the washing liquid is fed back to the wash tank 17, through theconduit means 15. Clean water or fresh water is delivered to the tank 17through a conduit means 18.

The fine filter 5 and the tank 17 are both supplied suitably with warm(hot) washing liquid from a water heater 19 which is connected to both aconduit means 20 for clean water and a conduit means 21 connected to themunicipal or local water mains.

Water that has been cleansed in the membrane filter 13 is passed fromsaid filter to a second buffer tank 22, either directly or through oneor more further filters. This buffer tank is connected to the municipaleffluent outlet 23, via a valve 24.

Water is taken from the second buffer tank 22 and delivered to thevehicle-washing plant 37 via a feed pump 25 and a conduit means 26.Water taken from the first-mentioned buffer tank 11 is also delivered tothe vehicle-washing plant, via a feed pump 27 and a first conduit means28. The system also includes a second conduit means 29 leading from thebuffer tank 11, wherein water is delivered from the buffer tank to thevehicle-washing plant through said second conduit means 29, via a feedpump 30.

Water that has been used in the vehicle-washing plant and which iscontaminated with oil is passed to a conventional slime or sludgeseparating tank 32, via a drain conduit means 31. Contaminated water ispumped from the tank 32 to the flocculating chamber 2 by means of theearlier mentioned feed pump 1, via conduit means 33.

Alternatively, the sludge separating tank 32 may be omitted. In thiscase, the conduit means 33 is also omitted and the drain conduit means31 leading from the vehicle-washing plant is, instead, connecteddirectly to the feed pump, as indicated by the broken line 34.

Thus in accordance with the first embodiment of the inventive method,water from the flocculating chamber 2 is cleansed in a first cleansingstage, by causing the water to flow through the collection filter 4 inwhich oil flocs are collected. The water will typically have an oilconcentration of 150 mg/l upstream of the collection filter 4. In asecond cleansing stage, the water from the collection filter 4 is causedto flow through the fine filter 5. This filter functions to reduce theoil content of the water to a level below roughly 50-75 mg/l. It ispreferred to lower the oil content to about 20 mg/l in the fine filter.

Essentially the same oil content is obtained downstream of the finefilter 5 in the case of the second embodiment.

In a third cleansing stage, the water from the fine filter 5 is causedto flow through the membrane filter 13 which functions to filter-out atleast 75% of the oil remaining in the water. It is possible with the aidof known membrane filters to filter-out about 98% of the remaining oil.At least a substantial part of at least the water cleansed in themembrane filter 13 is cycled back to the vehicle-washing plant, whileany remaining part is delivered to a municipal or local sewage network23. By substantial part is meant here approximately 50%-75%. However,the method includes the recycling of all water.

The water released to the municipal water network and reused after beingcleaned will have a very low oil content, below 4 mg/l.

This three-stage process, in which the oil content of the water islowered to a relatively low level in a second stage, enables a membranestage to be used as a final stage as regards cleansing water of its oilcontent. Because the oil content is relatively low after the secondstage, it is sufficient to use only one membrane filter of reasonablecost, this filter only needing to be cleaned once each calendar day.

Naturally, when a sludge separator is used, both the sludge separatorand the flocculating chamber must be cleansed from the oil that collectsin the system. A very large part of this oil, however, is removed fromthe collection filter 4.

According to one preferred embodiment, the fine filter 5 is cleaned atpredetermined time intervals, by causing the washing water to flow in adirection opposite to the direction of water-cleansing flow. The washingwater is led through to the flocculating chamber subsequent to cleaningthe fine filter.

According to another preferred embodiment, the membrane filter 13 iscleaned at predetermined time intervals by causing washing liquid toflow through the filter, preferably in a direction opposite to thewater-cleansing flow direction. The washing liquid may be a suitableliquid oil-solvent and water. The washing liquid is cycled back to thewash tank 17 after having passed through the membrane filter.

The water cleansed in the membrane filter 13 is passed to a secondbuffer tank 22. Water separated in the membrane filter 13, on the otherhand, is passed to the first buffer tank 11.

In the case of the embodiment just described, the system includes twobuffer tanks 11, 22 which respectively contain water that has beencleansed to different extents. Although the water in the first buffertank will contain some oil, the water can still be used conveniently towash a vehicle and to wash the underneath thereof. The water from thefirst buffer tank 11 is therefore passed to the actual vehicle-washingplant through a first recycling conduit means 29.

The second buffer tank 22 contains water that is essentially free fromoil and heavy metals, among other substances, and can be suitablyreleased as effluent to the municipal sewage network 23. This is alsowhat takes place to a degree which corresponds to water supplied to themunicipal drinking water system 21 minus the water that is carried awayby washed vehicles in those instances when not all water is recycled.The water in the second buffer tank 22 is far more suitable for thefinal rinsing of the vehicle than the water contained in the firstbuffer tank 11. The water in the second buffer tank is therefore passedto the vehicle-washing plant as rinsing water, via a second recyclingconduit means 26. It is through this conduit means 26 that freshmunicipal water is delivered to the plant.

A vehicle will entrain about 10 liters of water as it moves away afterbeing washed. Consequently, after each wash it is necessary to top upwith water from the municipal water mains in an amount corresponding atleast to this quantity. As before mentioned, according to oneembodiment, all water is recycled, wherein only about 10 liters of freshwater need be supplied to the system with each vehicle wash.

According to another preferred embodiment, the water is caused to flowbetween the membrane filter 13 and the second buffer tank 23 through aknown so-called R/O membrane filter 35 which operates in accordance withthe principle of reverse osmosis, such as to cleanse this water ofsalts, heavy metals and washing detergent. The water separated in theR/O membrane filter 35 is passed to the first buffer tank. Clean wateris passed to the second buffer tank 32 through a conduit means 36.De-salting of the water makes it well-suited for use as rinsing water ofa drinking water quality.

Thus, the present invention is highly effective in cleansing water,while at the same time recycling water with a low water consumption as aresult.

Although the invention has been described with reference to exemplifyingembodiments thereof, it will be apparent to the person skilled in thisart that other water flow couplings than those illustrated mayalternatively be used to achieve the characteristic features of thepresent invention.

The present invention is therefore not restricted to the aforedescribedembodiments, since modifications and variations can be made within thescope of the following claims.

What is claimed is:
 1. A method of extracting oil from oil-contaminatedwater, including spillage water from gasoline stations and water thatemanates from vehicle-washing plants, said method comprising the stepsof: passing oil-contaminated water from a source of oil-contaminatedwater through a collection filter in which oil and coarse particles arecollected in a first cleansing stage; passing filtered water from thecollecting filter through a fine filter to lower the oil content of thewater to a level below approximately 50-75 mg/l in a second cleansingstage, wherein the fine filter includes a mass based on sawdust that hasbeen treated to render the mass hydrophobic so that the mass is able toabsorb petroleum products without taking-up any appreciable amount ofwater, and wherein contaminated water is caused to pass through themass; passing treated water from the fine filter to flow through a firstmembrane filter to filter-off at least 75% of the oil remaining in thewater in a third cleansing stage; and recycling at least a part of atleast the water cleansed in the membrane filter back to the source ofthe oil-contaminated water; and discharging a residual part of saidwater from the third cleansing stage to a municipal sewage network.
 2. Amethod according to claim 1, wherein the mass in the fine filter isadmixed with a substance selected from the group consisting of activecarbon, aluminum selenite, synthetically produced selenite, andcombinations thereof.
 3. A method according to claim 1, wherein the massin the fine filter is replaced with fresh mass at pre-determined timeintervals.
 4. A method according to claim 1, including the steps offlocculating oil contained in the oil-contaminated water within aflocculating chamber upstream of the collection filter; and collectingflocculated oil in the collection filter.
 5. A method according in claim4, including the steps of cleaning the fine filter at predetermined timeintervals by causing washing water to flow in a direction opposite to awater-cleansing flow direction; and passing the washing water to theflocculating chamber after the washing water has passed through the finefilter.
 6. A method according to claim 1, including the steps ofcleaning the first membrane filter at predetermined time intervals bycausing washing liquid to flow through the first membrane filter; andpassing the washing liquid to a wash tank after passage of the washingliquid through the first membrane filter.
 7. A method according to claim1, including the steps of passing water cleansed in the first membranefilter to a second buffer tank from which water is taken to thevehicle-washing plant; and passing to the first buffer tank water thathas passed through the first membrane filter.
 8. A method according toclaim 1, including the steps of passing water that has passed throughthe first membrane filter to a reverse osmosis membrane filterpositioned between the first membrane filter and the second buffer tankto cleanse the water of salts; and passing water that has passed throughthe reverse osmosis membrane filter to the first buffer tank.
 9. Asystem for extracting oil from oil-contaminated water, includingspillage water from gasoline stations and water that emanates fromvehicle-washing plants, said system comprising a collection filter forreceiving oil-contaminated water and in which oil and coarse particlesare collected in a first cleansing stage; a fine filter to lower the oilcontent of the water to a level below approximately 50-75 mg/l in asecond cleansing stage, wherein the fine filter includes a mass based onsawdust that has been treated to render the mass hydrophobic, wherebythe mass is able to absorb petroleum products without taking-up anyappreciable amount of water; a first membrane filter to filter-off atleast 75% of the oil remaining in the water in a third cleansing stage;and a recycling conduit extending from the first membrane filter to thesource of the oil-contaminated water and through which at least asubstantial part of at least the water cleansed in the first membranefilter is recycled back to the source of the oil-contaminated waterwhile any residual part of said water from the third cleansing stage isdischarged to a municipal sewage network.
 10. A system according toclaim 9, wherein the mass in the fine filter includes a substanceselected from the group consisting of active carbon, aluminum selenite,synthetically produced selenite, and combinations thereof.
 11. A systemaccording to claim 9, including an oil-flocculating chamber upstream ofand in communication with the collection filter, said collection filterbeing adapted to collect oil flocs.
 12. A system according to claim 9,including a first buffer tank between the fine filter and the firstmembrane filter, the water cleansed in the fine filter being piped tosaid first buffer tank through a conduit means; wherein the first buffertank is connected to the source of the oil-contaminated water by arecycling conduit.
 13. A system according to claim 12, including asecond buffer tank between and in communication with the first membranefilter and a second recycling conduit means, wherein the water cleansedin the first membrane filter is piped to the second buffer tank througha conduit means.
 14. A system according to claim 9, including a reverseosmosis membrane filter positioned between and in communication with thefirst membrane filter and the second buffer tank for extracting saltsfrom the water.
 15. A system according to claim 9, wherein a part of thefirst membrane filter that contains separated water is connected to thefirst buffer tank.